The ever-increasing regulation of emissions from vehicles has been met by a combination of engineering design, including sophisticated engine management, and a variety of catalytic aftertreatment devices.
In the case of exhaust gases from compression ignition (hereafter called “diesel”) engines, the regulated emissions are carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx) and particulates. Particulates, although often simply called “soot” are complex solids of which carbon particles form a major part. At one time, the emission of particulates was the characteristic “smoke” issued by diesel engines, particularly when operating under heavy load such as during hill climbing. The carbon particles also carry varying amounts of absorbed or adsorbed hydrocarbons and oxidation by-products.
A Diesel Oxidation Catalyst (DOC) was the first catalytic aftertreatment device introduced for light duty diesel vehicles. A DOC comprises a catalyst, almost exclusively based on a Platinum Group Metal (PGM), particularly platinum, optionally in combination with another PGM and/or a catalytic base metal, deposited on a high surface area flow-through catalyst substrate. A substrate unit is colloquially called a “brick”. The DOC is effective to oxidise CO and HC, and to reduce the weight of particulates, and for some years a DOC was adequate to meet the regulations for light duty diesel emissions.
As higher standards for emission control from diesel vehicles were introduced, it became necessary to deal with particulates. Diesel engines generally produce a lower level of NOx than spark-ignition or gasoline engines, and diesel engines can be engineered to decrease particulates, although at the expense of an increase of NOx, or vice versa.
Under the regulations current in Europe and being introduced in USA, state of the art diesel vehicles now have a combination of a DOC and a soot filter, and may have a NOx reducing system also. The soot filter is often, but not exclusively, a Catalysed Soot Filter (CSF), and although these may differ in detail, they generally incorporate a ceramic wall flow filter or, less frequently, a sintered metal filter. The catalysed filter carries a deposit of a soot oxidation catalyst, such as a PGM and an alkali metal or alkaline earth metal combination.
Such filters are capable of meeting the regulations for particulate emissions. Although during normal drive cycles involving a high speed section such filters are not plagued with accumulation of soot, real life scenarios, for example in cities, may include extended periods of low-speed driving and/or extended idling periods, during which soot can accumulate in the filter but the exhaust gas temperatures are insufficient for the catalytic oxidation of the soot. The blocking of either the face of the filter or accumulation of soot within the passageways in the filter, may be such as to cause an increase of back pressure to the extent that engine efficiency is badly affected, or in an extreme case the engine itself is damaged. It is therefore necessary to provide filter regeneration of some form to clear the filter.
Passive regeneration involves running the vehicle at a sufficiently high speed that the exhaust gases reach a temperature at which accumulated soot begins to oxidise.
The alternative of active regeneration is described for example in GB 2406803A. Fuel in the form of supplementary HC in the exhaust gas is supplied to the DOC, where it combusts and raises the temperature of the exhaust gases to the point where catalytic combustion of the accumulated soot is initiated on the catalytic soot filter. GB 2406803A (Johnson Matthey) concerning active regeneration discloses a DOC having a Pt-component in an upstream end of the brick with a Pd-component in the downstream end of the brick. The only other disclosure of dimensions states “The substantially Pd-free Pt containing zone can be up to half of the length of the substrate monolith or of a “stripe”-type dimension.” It is to be noted that the purpose of the differing compositions arises from sulfation arising from sulphur-containing diesel fuels. Although in European countries current diesel fuels have low or ultra-low sulphur contents, there remain issues with the higher sulphur levels found in many other countries.
WO 00/29726 (Engelhard) describes in one embodiment a flow-through catalyst or DOC (confusingly described in some places as a second catalyst) upstream of a catalysed soot filter. The DOC is of uniform composition, but the CSF may have a 4 inch Pt-enriched end. In some tests, the DOC is tested together with the CSF, and in some experiments the CSF is used alone. In some of the CSF experiments, the Pt-enriched end is upstream, in some experiments the Pt-enriched end is downstream. This early CSF disclosure does not contemplate active regeneration of the CSF.
WO 2009/005910 (Cummins) describes an exhaust aftertreatment device in which the front face of a brick has a physical form or a chemical coating effective to prevent “blinding” or blockage or face-plugging of a brick exposed to a diesel particulate-containing exhaust gas stream. When a chemical coating is used, it is plainly stated that this is intended to be carried on the front face of the brick and does not extend into the fluid passageways of the brick.
WO 2007/077462 (Johnson Matthey) discloses a triple-zoned DOC, intended to improve upon the performance of a DOC having uniform composition or a DOC having a single zone of higher Pt loading at the upstream end of the brick. No active regeneration system is disclosed, so that there is no periodic enrichment of exhaust gas. Although the invention is intended to deal with a “lit-out” problem, that problem is created on cooler parts of the New European Drive Cycle. The inventors of WO 2007/077462 discovered that a uniform loading of PGM on the DOC, or a DOC with a relatively high loading on a front section, exhibit a tendency to light-out or extinguish. The solution advocated in WO 2007/077462 is to incorporate a third, higher loading, zone at the downstream end of the DOC. The problem dealt with is different from that of the present invention.